I think you can compare tagging with the old
<meta name="Keywords" content="ajax, web2.0" />
tags.

However, services like del.icio.us have collected tags from numerous websites and they therefore serve as a kind of a "hosted meta tag" solution. As the major blog all support del.icio.us, this company is probably the biggest player on the tagging market. Yahoo will base their seo decisions and siterank algoritms on the rank of sites in del.icio.us.

Google launched "Google Co-op" a similar service to del.icio.us, so it is probable that future algoritms of the big search engines will take tagging or social bookmarking into consideration.

So what's the big difference between social bookmarking using tags or just putting meta tags into the header of your website ? Well, the big advantage is that the end-user has the power like in a democration. If many people like your site and they bookmark it via a service like del.icio.us or google co-op, your site will increase in its popularity. It is only logical that seacrhengines will rank your site better in this case.